The present invention relates to the reformatting of variable rate data for communication to an external device at a fixed rate. The invention has particular application in the provision of an extensible interface for a communications terminal such as a set top box or integrated receiver decoder (IRD) for digital television, in which the data stream received by the terminal carries data for use by an external device ("external data") in addition to television and access control signals.
Digital transmission of television signals can deliver video and audio services of much higher quality than analog techniques. Digital transmission schemes are particularly advantageous for signals that are broadcast via a cable television network or by satellite to cable television affiliates and/or directly to home satellite television receivers. Examples of such schemes are the MPEG-2 data transmission standard and the DigiCipher.RTM. II standard proprietary to General Instrument Corporation of Chicago, Ill., U.S.A., the assignee of the present invention. The DigiCipher.RTM. II standard extends the MPEG-2 systems and video standards, which are widely known and recognized as transport and video compression specifications specified by the International Standards Organization (ISO) in Document series ISO 13818. The MPEG-2 specification's systems "layer" provides a transmission medium independent coding technique to build bitstreams containing one or more MPEG programs. The MPEG coding technique uses a formal grammar ("syntax") and a set of semantic rules for the construction of bitstreams. The syntax and semantic rules include provisions for demultiplexing, clock recovery, elementary stream synchronization and error handling. The syntax and semantics of the MPEG-2 transport stream are defined in International Organisation for Standardisation, ISO/IEC 13818-1, International Standard, 1994 entitled "Generic Coding of Moving Pictures and Associated Audio: Systems," recommendation H.222, incorporated herein by reference. It is expected that digital television transmitter and receiver systems will replace existing analog systems just as digital compact discs have largely replaced analog phonograph records in the audio industry.
An advantage of digital transmission techniques is that the television signals can be compressed using various well known compression techniques in order to free up bandwidth in the cable or satellite television spectrum. This bandwidth can be used to provide additional services, such as additional television channels and/or the communication of external data that may or may not be related to the television services. Currently, television, telephone, personal computer (PC) and other technologies are beginning to merge. In the interim, it is desirable to allow products using such technologies to communicate with one another through an efficient and low cost interface.
In particular, it would be advantageous to provide an external data interface port on "set top" boxes that receive television signals from satellite and cable television systems. Such an interface port would enable the communication of data which is carried with television program services but is independent of said services to external peripherals, such as a personal computer, video player/recorder, video game, or the like. It would be particularly advantageous if such an interface port provided for the communication of data to the external peripheral component at virtually any desired fixed rate. Moreover, it would be advantageous for such an interface to provide for the reformatting of variable rate data received from a cable or satellite television system into fixed rate data in an efficient and low cost manner.
The present invention provides an interface having the aforementioned and other advantages. In particular, the interface of the present invention allows output at a fixed rate no matter what the information rate. This feature enables a simple and inexpensive recovery circuit to be implemented using a simple and inexpensive connector, such as a miniature headphone jack or other well known connector. The circuit is also substantially free of electromagnetic interference (EMI). Data is carried over the interface in a differential mode, providing good noise immunity and a two wire interface that is not sensitive to wiring polarity. Since the information rate is independent of the receiver clock, which establishes the fixed output rate, the transmission rate can be used as a time reference. Thus, the external device can recover the clock very easily despite the variable information rate. Since no DC path is required between the connector and the chassis of the receiver, troublesome ground loops are eliminated. Additional advantages will be apparent from the following disclosure.